Adblock was the very first ad blocking extension, after a while the author stopped developing it, so another person took up the Adblock Plus project.

That developed untill version 0.5.x when yet another Adblock Plus project was started, aiming at a more optimised and resource intenseless extension. The fist Adblock has resumed development at about the same time, and in order to avoid confusion (which was already enough) the developer of the first Adblock Plus joined the original Adblock's development team.

The current Adblock Plus 0.7 is the second Adblock Plus project, with many new features and better memory management.
The current Adblock is a renewed version of the original one, it continues its line of development and I believe focouses on features more than memory management (however that is being worked upon too).

Featurewise, just compare the two's feature lists and/or changelog.

I would recommend Adblock Plus as it's more efficient and has some features that the original Adblock doesnt have.

* Added translations: Brazilian Portuguese, Dutch, Chinese Simplified, French, Danish, British English, Portuguese, Swedish, Chinese Traditional * Preferences dialog: Made filter list a multi-column tree * Preferences dialog: Added filter search (also find-as-you-type if enabled in browser) * Preferences dialog: Filters and subscriptions can be reordered via drag and drop * Preferences dialog: Added copy and paste for filters * Preferences dialog: The list can be sorted by any column now * Preferences dialog: Showing element hiding rules and invalid rules (mainly wrong regular expressions) in separate groups * Preferences dialog: Element hiding rules are shown as invalid when used in a browser that doesn't support them (Gecko 1.7 and lower) * Preferences dialog: Added an Edit menu * Preferences dialog: Added Help button (opens FAQ) * Sidebar: Added column sorting * Sidebar: Can be filtered by address or type text * Sidebar: Added filter and state columns (not shown by default) * Sidebar: Tooltip showing more information now * Sidebar: Images and background images are displayed in the tooltip for easier identifying (can be disabled via preference extensions.adblockplus.previewimages) * Sidebar: Splitting up long addresses into multiple rows in the tooltip * Sidebar: Current state is saved when detaching/reattaching * Sidebar: Making sure to scroll to the item when flashing its borders (can be disabled via preference extensions.adblockplus.flash_scrolltoitem) * Add subscription dialog: Made subscription title first input field * Add subscription dialog: Added a dropdown list with a few suggestions for subscriptions * Whitelisting via menu now allows to whitelist a specific page, not all its subpages as well * Added an option to show/hide toolbar icon * "Show in status bar" switched off by default * Toolbar icon now visible in Mozilla and SeaMonkey in the personal toolbar * Remembering now whether Preferences dialog/detached sidebar were maximized and restoring state when reopened * Preferences extensions.adblockplus.defaulttoolbaraction and extensions.adblockplus.defaultstatusbaraction now define the action for left-click on toolbar/status bar icon * Preferences extensions.adblockplus.sidebar_key, extensions.adblockplus.setting_key and extensions.adblockplus.enable_key now define the keyboard shortcut for opening/closing sidebar, opening preferences and enabling/disabling * Selecting active locale on startup in Mozilla and SeaMonkey to fix installer's bad choice * Fixed: Don't allow to add filters via "Disable on ..." while the Preferences dialog is open * Fixed: Cannot open Preferences when either About or Subscription dialog is open * Fixed: When switching to an already open Preferences dialog suggestion isn't filled in and the filter is not selected * Fixed: Spaces shouldn't be stripped from subscription address/file name * Fixed: Detached sidebar wouldn't save state in Firefox 1.0/Mozilla 1.7 * Allowed blocking background images via context menu * Made it possible to drag images/links to the toolbar icon as well, not just the status bar

Core

* Only exception rules starting with @@http:// or @@https:// will whitelist entire pages, other rules apply to individual items only * Saving daily backups of patterns.ini (max. 5 backups by default) * %VERSION% in subscription addresses is replaced by current Adblock Plus version when updating, e.g. ABP0.7 * Added interface nsIAdblockPlus to be used by other extensions (for example to manage external subscriptions) * Moved several hardcoded lists to preferences * Choosing which schemes can be blocked with a whitelist (extensions.adblockplus.whitelistedschemes), not a blacklist * Fixed: Blockable items list would show all visited pages, not just the current (mainly on Linux) * Fixed: Memory leak happening with Gecko 1.8 (not Gecko 1.9) when "Show tabs on Flash and Java" is activated * Fixed: Performance issues on pages with many occurences of the same address * Fixed: Switching profiles at runtime in Mozilla and SeaMonkey isn't handled properly * Fixed: Error message "secureLookup(insecDoc, 'createEvent') is not a function" * Filters starting with http:// or https:// no longer match only at the beginning of the address * Starting filters with | makes them match only at the beginning of the address, finishing them with | makes them match only at the end * Fixed file permission problems when installing on Linux in Mozilla Suite/SeaMonkey * Made sure there is no way for web sites to detect Adblock Plus

Detailed changelog for Adblock Plus 0.6.1

The following lists the major changes compared to Adblock Plus 0.6:New features

* Filter subscriptions (formerly "synchronization") * Sidebar can be detached (button in the upper-right corner) * Showing the filter that has been applied in the sidebar's tooltip * Double-click on a blocked/whitelisted item in the sidebar selects the filter in the Preferences dialog automatically * Toolbar icon in Firefox (customizable — can be moved around or removed) * Status bar icon has a context menu (same as the toolbar icon's menu) * Middle-click on the icon in the toolbar or status bar disables/re-enables Adblock Plus * Added minimal support for "DIV blocking" (see FAQ for details)

User interface

* Added translations: German, Spanish, Italian, Russian, Polish, Slovak, Lithuanian, Romanian, Finnish, Turkish, Japanese * Made Adblock Plus show up as an icon in the status bar (optional now, can be removed via Options) * Mozilla Suite/Seamonkey: Added the sidebar and made it behave in the same way as in Firefox (opening via menu or Ctrl+Shift+B works) * Removed limit for tooltip size in the sidebar * Sidebar now tells that a page is whitelisted instead of "no blockable items" * Sidebar always updating properly now even when going to a page without blockable items * Whitelisting a site or a single page possible via toolbar icon's menu * Preferences dialog: Filters are grouped by type * Preferences dialog: Added a warning when Adblock Plus is disabled * Preferences dialog: "Sort alphabetically" only sorts visually and any sorting applies immediately * Preferences dialog: Sorting ignores non-alphabetical characters * Preferences dialog: Multiple selection possible * Preferences dialog: Reordering of filters via context menu or Ctrl-Up/Down (not when sorting is enabled) * Preferences dialog: Blocked items in the suggestions list can also be selected now * Preferences dialog: Using platform's line breaks for exported filters * Preferences dialog: Added option to cancel the warning when importing filters * Preferences dialog: OK doesn't perform "Add filter" automatically any more * Preferences dialog: Moved parts of the "Options" menu into the new "Filters" menu * Preferences dialog: Made menu items and OK button disable when necessary * Preferences dialog: Options apply immediately now * Preferences dialog: Line height decreased for the filters list and suggestions dropdown

Core

* Made whitelisting apply to individual items as well, not only complete web pages * Improved handling of frames (fixed: blocked frames wouldn't show up in the sidebar) * When the option "block banner links" is used those links are treated as a separate element and can be blocked via context menu * Optionally blocking ads in local pages now (enabled by default, some RSS readers need this) * Uninstalling Adblock/Adblock Plus 0.5 automatically now (advising manual uninstall in Mozilla Suite and Seamonkey) * Improved performance

Detailed changelog for Adblock Plus 0.6

The following roughly lists the changes compared to Adblock 0.5.2.039:User interface

* Clicking "Adblock" in context menu, main menu and status bar opens three different dialogs - FIXED * Preferences dialog has buttons OK and Cancel instead of Done and Revert now * Filter list in the Preferences dialog is a buggy fork of the default listbox widget - FIXED * Menu in the Preferences dialog is the standard menu widget now instead of toolbarbuttons+popups * Global key handling routine in the Preferences dialog untangled, making use of preventDefault to ensure correct key handling * ENTER key is now interpreted like an OK click in the Preferences dialog (ESC key handling also fixed, see previous point) * Flashing the selected/entered URLs works everywhere now * Selecting "Keep list sorted" in the Preferences dialog menu applies immediately now instead of displaying a message asking to close and reopen the dialog * Adding one menu item to the Tools menu instead of a submenu with three menu items * Adding two shortcut keys to the browser window instead of five * Not overriding CSS of the current theme any more

Security

* No data stored in DOM or otherwise accessible from content scripts * Limit interaction with DOM to a minimum * Secure DOM access without XPCNativeWrappers

New features

* Adblock sidebar (Firefox only) shows blockable URLs in the currently selected tab dynamically * Whitelisting, compatible with Adblock Plus. TODO: Better whitelisting + UI * About dialog, displaying the installed version number as reported by the extension manager (no need to change it manually with every release) * Added three default filters as examples (not referring to any specific sites) * Dragging URLs to be blocked to the status bar (no idea what this is good for but maybe that's what the removed drag&drop code was meant for) * Enabling/disabling Adblock with a drag gesture on the status bar (another guess what this drag&drop code was meant to do) * Localizations are possible now

Removed features

* Decide what filters can't possibly apply based on host name - my original implementation was meant as a speedup, it has been severely broken since and turned into a slowdown. The way Adblock works now this speedup can't be used any more. * Some weird non functional drag&drop code - can only guess what this was supposed to do, see "new features" above * ObjectOverride - was replacing objects with clickable DIVs to allow blocking them, very easy to run into big trouble here. Definitely the wrong way to do this. * QuickBlock - I couldn't figure out what this did, not even by looking at the code. Michael McDonald told me now (it removes images temporarily on Ctrl-Shift-DblClick) but I don't think more than 10 people know. * Site blocking - very buggy and an absolute non-feature. Blocking a page that was explicitly requested - what does this have to do with ad blocking? * Map area disabling - no idea how this is supposed to help block ads. If there is a reason behind this, there is no way for users to figure it out. * Removing conflicting shortcut keys from the menu - that's really not a nice thing to do, extensions shouldn't do this. * Extension manager window overlay to display Adblock version number in a different format - besides the fact that an extension simply shouldn't do this, the standard version number format is still visible in quite a few places. * Installation of a component into the profile in browser versions that don't support this - my big ugly hack to do this was only meant as a temporary measure until proper support for installing components into the profile was added which happened long ago. * Very dangerous uninstall code for Mozilla Suite

Toward the end is the first version Wladimir Palant did on adblock plus. He had removed some things that upset some people, but in later version those things were added back in other ways that worked out with better performance. And lots of extra things that no other version of adblock has. Almost seems I should have posted the log the other way around, but I think the latest updates should be at the top.

PS sorry so long.. lots of changes in a short time. This is one active developer..and all the changes are for the better. IMHO